In a typical marine seismic acquisition, a vessel tows a source array and several receiver arrays commonly referred to as “streamers”. In a conventional towed-streamer marine acquisition configuration, shallow sources and shallow cables increase the high-frequency content of the seismic data needed for resolution. However, shallow sources and shallow cables attenuate the low frequencies, which are necessary for stratigraphic and structural inversion, and for imaging deep objectives.
Towing shallow also makes the data more susceptible to environmental noise. In contrast, deep sources and deep streamers enhance the low frequencies, but attenuate the high frequencies. In addition, the data recorded via a deep tow have a higher signal-to-noise ratio (S/N) due to the more benign towing environment. A conventional towed-streamer survey design therefore, attempts to balance these conflicting aspects to arrive at a tow depth for the sources and cables that optimizes the bandwidth and signal-to-noise ratio of the data for a specific target depth or two-way travel time, often at the expense of other shallower or deeper objectives.
An over/under, towed-streamer configuration is a method of acquiring seismic data where cables are typically towed in pairs at two different cable depths, with one cable vertically above the other. The depths of these paired cables are typically significantly deeper than would be used for a conventional towed-streamer configuration.
In conjunction with these paired cables, it is possible to acquire data with paired sources at two differing source depths.
The seismic data recorded by the over/under towed-streamer configuration are combined in data processing into a single dataset that has the high-frequency characteristics of conventional data recorded at a shallow towing depth and the low frequency characteristics of conventional data recorded at a deeper towing depth. This combination process is commonly referred to in the geophysical literature as deghosting, as it effectively removes the so-called ghost notches from the receiver response.
The sea surface ghost is an important phenomenon that determines the signal-to-noise ratio of marine seismic data. Due to the negative sea surface reflection, a ghost “notch” manifests itself in the seismic response spectrum at specific frequencies where there is a destructive interference of the oppositely travelling wavefields. At normal incidence (vertically travelling waves) the ghost notch frequency is given by:fnotch=nc/2h,  [1]where integer n≧0, h is the streamer depth and c the water velocity. Of particular importance is the zeroth ghost notch at 0 Hz.
Over-under acquisition overcomes this problem for higher (n>0) ghost notches by adding the signals recorded by streamers at different depths with each other. In that way signal recorded fill the ghost notch of the over streamer and vice versa.
The interference spectrum of the upgoing and downgoing wavefield includes ghost maxima at frequencies of constructive interference between the up- and downgoing wavefield. The interference or ghost response spectrum reaches such maxima at the mid-frequency between two neighboring ghost notches
The above process is described in greater detail by:
Hill, David; Combee, Leendert; Bacon, John in: “Over/under acquisition and data processing: the next quantum leap in seismic technology?” First Break Vol. 24, June 2006, pp 81-95.
Other references to over/under seismic acquisition methods can be found for example in the U.S. Pat. Nos. 4,992,991 and 6,493,636, both of which describe over/under streamer configuration with a different numbers of streamers at different depth levels.
A significant disadvantage of conventional over/under acquisition methods is that it requires twice the number of streamers compared to a conventional shallow towed acquisition. The economics of a 3D over-under survey thus compares unfavorably with a conventional 3D survey. Given that a vessel's capacity of towing streamers is fixed, the width of the spread is necessarily halved resulting in a doubling of the number of sail-lines.